Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
FIG. 1 depicts a typical front wheel drive powertrain arrangement. Flow of mechanical power is shown by solid lines. Power is provided by internal combustion engine 10. Transmission input shaft 12 transmits power from the crankshaft of engine 10 to torque converter 14. Torque converter 14 permits the engine to idle while the vehicle is stationary. Torque converter 14 transmits the power to gearbox 16 via turbine shaft 18. In some operating conditions, torque converter 14 may decreases shaft speed and increase shaft torque. Gearbox 16 adjusts the speed and torque according to current vehicle requirements and transmits the power to differential 20. Differential 20 transmits the power to left and right wheels 22 and 24 while permitting slight speed differences when the vehicle turns a corner.
Torque converter 14 requires a supply of fluid to establish a hydrodynamic torque flow path and may also require a supply of pressurized fluid to engage a lock-up clutch. Gearbox 16 requires a supply of fluid for lubrication and cooling. The speed ratio of gearbox 16 is controlled by supplying fluid at controlled pressures to a collection of shift elements such as brakes and clutches. The fluid for these purposes is supplied by pump 26 and regulated by valve body 28. Pump 26 is mechanically driven by the transmission input shaft 12. Pump 26 draws fluid from a sump 30 and provides the fluid to valve body 28 at an elevated pressure called line pressure. Valve body 28 distributes the fluid into a number of separate circuits directed to torque converter 14 and gearbox 16 at various pressures that may be controlled to pressures less than line pressure. In some cases, the valve body directs flow through a cooler 32. Fluid drains from the gearbox and valve body back to the sump 30. Components within the dotted box 34 are typically contained within a common housing and called a transaxle.